The purpose of this project is to contribute to the understanding of hormone action on metabolic processes in normal breast tissue and to understand the failure of or aberrant response to hormones by hyperplastic or neoplastic breast tissue. Among the topics studied are the response of mammary tissue in culture to hormones in terms of nucleic acid and protein synthesis. Transfer RNAs from mammary glands of mice were isolated and separated by two-dimensional electrophoresis to determine changes that may occur in the developing tissue in vivo and in vitro. Quantitative and qualitative differences in isoacceptor patterns of transfer RNAs obtained from mammary tissues of mice in early and late pregnancy were observed. In contrast, patterns from late pregnant and lactating tissue were very similar. To determine whether patterns of tRNAs might be altered in concert with synthesis of milk protein mRNAs, explants from mammary glands of mice in mid-pregnancy were incubated in synthetic medium with insulin and hydrocortisone in the presence or absence of prolactin. Patterns of tRNAs from prolactin stimulated and unstimulated explants labeled with 32-Pi were similar to tRNAs from late pregnant tissue and to each other.